Laser Actuation of Cantilevers for Picometre Amplitude Dynamic
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OPEN Laser Actuation of Cantilevers for SUBJECT AREAS: Picometre Amplitude Dynamic Force CHARACTERIZATION AND ANALYTICAL Microscopy TECHNIQUES POLYMERS Drew R. Evans1,2, Ponlawat Tayati1, Hongjie An1, Ping Koy Lam3, Vincent S. J. Craig1 & Tim J. Senden1 CHEMICAL PHYSICS 1Department of Applied Mathematics, Research School of Physics and Engineering, The Australian National University, Canberra, ACT, 0200, Australia, 2Thin Film Coatings Group, Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Received 3 Australia, Department of Quantum Science, Research School of Physics and Engineering, The Australian National University, 24 March 2014 Canberra, ACT, 0200, Australia. Accepted 5 June 2014 As nanoscale and molecular devices become reality, the ability to probe materials on these scales is Published increasing in importance. To address this, we have developed a dynamic force microscopy technique where 4 July 2014 the flexure of the microcantilever is excited using an intensity modulated laser beam to achieve modulation on the picoscale. The flexure arises from thermally induced bending through differential expansion and the conservation of momentum when the photons are reflected and absorbed by the cantilever. In this study, we investigated the photothermal and photon pressure responses of monolithic and layered cantilevers using a Correspondence and modulated laser in air and immersed in water. The developed photon actuation technique is applied to the requests for materials stretching of single polymer chains. should be addressed to T.J.S. (tim.senden@ he Atomic Force Microscope (AFM) is widely used for high resolution imaging of surfaces, performing force anu.edu.au) or analysis, surface characterization1,2, and manipulating molecular systems ranging from DNA3 and motor V.S.J.C. (vince.craig@ proteins to classical polymer chains4–7. The theoretical understanding of performing work on molecular T 8–10 anu.edu.au) systems is well established , however, the experimental ability to perform nano-mechanical work on molecular systems has been problematic. The difficulty lies in the detection of changes in low energy regimes where the work performed on a system is comparable in magnitude to that of the work done by thermal fluctuations, due to Brownian motion. In this sense, the accuracy of AFM measurement at the molecular scale is said to be thermal fluctuation limited11–13. Minimizing the effect of thermal noise can be achieved either by averaging many mea- surements (although this has some inherent issues14) or by modulating the sensor such that the contribution of thermal noise at the modulation frequency is small in comparison to the total measureable signal. The latter principle is at the heart of dynamic force microscopy. A key feature of dynamic AFM is the method used to produce the cantilever excitation. Usually, a piezo-electric actuator or magnetic particle are used to generate acoustic and magnetic excitation15,16, with the excitation method influencing the mode of vibration17. Recently it has been argued that piezoacoustic excitation of canti- levers can preclude accurate interpretation of data18. Optical excitation of AFM cantilevers has been achieved by modulating the intensity of a laser impinging on the cantilever both in air19–21 and in liquid22,23. This method produces a frequency response unaffected by spurious contributions of noise from mechanical coupling through liquids, thus providing an opportunity to explore details of hydrodynamic and fluid systems. In this study, we use a modulated blue laser (wavelength: 405 nm) to excite an AFM cantilever. There are two ways to induce cantilever flexure via a laser: thermal heating, which leads to differential expansion and photon pressure. The latter was theoretically predicted by James Clerk Maxwell in the 1860’s24, whereby light (or indeed any radiation) exerts a small force on the surface it impinges. The relationship between the power of the impinging radiation and the exerted force is given by Equation 125. 2P zP F ~ R A ð1Þ photon c Where FPhoton is the photon pressure force (N), PR is the fraction of photon power reflected (W) from the surface, PA is the fraction of absorbed photon power (W) and c is the speed of light (m/s). It must be noted here that the functional form of Equation 1 for a cantilever would require knowledge of reflectivity and absorptivity of the cantilever, which varies with the laser wavelength and optical geometry (angle of incidence). SCIENTIFIC REPORTS | 4 : 5567 | DOI: 10.1038/srep05567 1 www.nature.com/scientificreports Methods CSC-38 are rectangular silicon cantilevers with an aluminium coating (Mikromasch). When a load is applied to a cantilever, it will deflect until a force balance is reached Water used was purified using a MilliQ Gradient system. Silicon substrates were between the applied load and restoring force of the cantilever. Using the framework cleaned using a RF frequency water plasma (10 W 45 s). PNIPAM (MW 5 5035, 27 outlined in26, the photon pressure induced force per unit length of the cantilever, can Polydispersity Index (PI) 5 2.1) was synthesised according to Zhou et al. , preci- 13 be related to the measured deflection of the cantilever under a distributed load by pitated twice from acetone/n-hexane and confirmed with C-NMR. Equation 2, (2P zP )(a2{3az3) Results and Discussion dopt ~ R A ð2Þ 3cka(a{2)2 The frequency response spectrum of a laser modulated AFM can- tilever is complex. The photothermal amplitude is largest at low where, dopt is the end load calibrated measured deflection from the optical lever (m) [ie the standard detection system used on many commercial instruments]; a is the modulation frequencies as the cantilever is able to cool and heat on normalised measurement position (a 5 x/L where x is the distance of measurement the time scale of the driving frequency and it oscillates between laser spot from the base of the cantilever and L is the length of the cantilever); and k is deflection positions corresponding to the warm and cool states. As the spring constant of the cantilever (N/m). We use this equation to calculate the the frequency is increased, the time available for cooling and heating deflection measured using the optical lever technique for a given total laser power, and compare it to the observed deflection. For static measurements, dopt is a deflection is decreased and consequently the amplitude of oscillation decreases. of the cantilever caused by continuous laser illumination. To perform dynamic At sufficiently high modulation frequencies the cantilever effectively measurements, the cantilever is modulated and the response of the cantilever is remains at a constant temperature and oscillation due to the photo- observed at the actuated frequency. Given we are using a dynamic method, the thermal effect becomes negligible. In comparison the contribution amplitude response of the cantilever due to the photon pressure can be derived from Equation 2 as, due to photon pressure is not expected to show any frequency dependence. As the excitation frequency increases other bending 1 (2P zP )(a2{3az3) ARMS~ pffiffiffi R A ð3Þ modes can result in broadening of the resonance peak. Evidently { 2 2 2 3cka(a 2) the contribution due to photon pressure, will be most evident at The amplitude response due to the photon pressure can be calculated for a cantilever frequencies sufficiently large that the photothermal oscillation is with known spring constant, measurement position, and the reflected and absorbed reduced but below frequencies where the resonance of the cantilever photon power. Here we use a modulated laser beam, in the arrangement depicted in is evident. We call this frequency window the photon pressure Figure 1, to fully illuminate the underside of an AFM cantilever, causing it to oscillate. When light impinges on the AFM cantilever a portion of the light is absorbed by the regime. cantilever, resulting in heating, the remainder is reflected. By conservation of The frequency response spectra of a Multi75-Al cantilever with momentum, there is a net transfer of momentum from the photons to the cantilever, and without laser excitation is shown in Figure 2. Without the addi- causing the cantilever to deflect in response. To demonstrate this technique, an AFM tional excitation, the (thermally driven) frequency spectrum shows (Research MFP-3D, Santa Barbara, CA) was modified to incorporate a modulated laser unit (Coherent Compass Laser, l 5 405 nm) where the modulation signal is the fundamental resonance frequency of a cantilever to be 63.6 kHz produced from an arbitrary function generator (Hewlett Packard/Agilent with quality factor, Q 5 155. The laser actuated spectrum yields HP33120A). The modulated laser beam was coupled to the AFM via an inverted approximately the same fundamental resonance frequency, however microscope (Nikon Eclipse TE 2000-U). The dynamic deflection amplitude was the peak is broadened. It is worth noting that the laser actuated adjusted by varying the total incident laser power using neutral density (ND) filters. The power output of the laser was measured using a power meter (Field master GS, spectrum does not show any additional spurious oscillation modes Coherent). Cantilever deflection was detected using the inbuilt conventional optics of which would otherwise appear as artefacts in the spectrum, implying the Asylum AFM. The oscillatory component of the deflection signal was isolated no other vibrational mode is excited, only the first flexural vibrational